1. Field of the Invention
The present invention relates to a valve device used in an injection molding machine for producing plastic resin products, and more particularly to such a valve device which includes a piston defined therein with a vertical cylindrical space having a lower portion partially receiving an injection nozzle adapted to inject molten resin into the cavity of a mold, and an upper portion partially receiving a connector adapted to connect the injection nozzle to a cylinder serving to supply molten resin, the piston also having a valve pin carrying portion configured to vertically support a valve pin in such a fashion that the valve pin moves vertically. In this valve device, it is possible to not only obtain an optimum stroke of the piston, but also to minimize the size of the valve device. Therefore, the valve device is proper for the molding of small-size resin products. In addition, the piston has a configuration capable of preventing heat, which is generated from heaters installed to prevent molten resin from being solidified during its feeding, from being directly transferred to packings which are installed to provide a seal effect along inner and outer surfaces of the piston, thereby preventing the packings from being damaged.
2. Description of the Prior Art
Typically, injection molding machines are used in the manufacture of plastic resin products. In such injection molding machines, a resin material is charged into a cylinder which, in turn, melts the charged resin material. The melted resin material is fed to a runner and then injected into the cavity of a mold, consisting of upper and lower mold portions, through an injection nozzle communicating with the runner. After filling the molten resin material in the mold cavity, a resin solidification is carried out. Following the resin solidification, the upper and lower mold portions of the mold are separated from each other to remove a molded product. In such injection molding machines, a resin residue, called a sprue, exists in a portion of the cavity of the mold near the injection port of the injection nozzle after the runner is separated from the mold upon a completion of the filling of the molten resin in the cavity. Such a sprue should be removed by a subsequent process. In order to eliminate such a drawback, a valve device having a configuration capable of eliminating the formation of a sprue has been proposed. For example, such a valve device is disclosed in Korean Utility Model No. 96556 issued to the applicant of the present invention. In the valve device disclosed in the utility model, a cylinder provided with a hydraulically or pneumatically-operating piston is arranged in the runner. A valve pin is coupled to the piston so that it serves to open/close the injection port of an injection nozzle in accordance with a upward/downward movement of the piston.
In this valve device, the hydraulic or pneumatic cylinder is arranged over a manifold, which is disposed in the runner, so that a molten resin is injected into the cavity of a mold through the manifold without using any resin channel. Such a configuration is appropriately used in the case in which a plurality of gates, adapted to inject molten resin into the cavity of the mold, are needed to mold large-size resin products. However, this valve device has a problem in that it is impossible to mold resin products of a small size requiring the use of a mold having a single gate.
In order to solve this problem, another valve device proper for the molding of small-size resin products has been proposed. For example, such a valve device is disclosed in Korean Utility Model No. 116226 (Publication No. 97-6954) issued to the applicant of the present invention. However, this valve device also involves various drawbacks. For example, this valve device is bulky as compared to an injection nozzle used. This is because a hydraulic or pneumatic cylinder, which is included in the valve device to move a valve pin adapted to open and close the injection nozzle, is arranged in one side of the injection nozzle. Furthermore, the valve pin is indirectly connected to a piston included in the hydraulic or pneumatic cylinder via a guide link arranged over the injection nozzle. For this reason, the valve pin may not move in a vertical direction. The valve pin may come into frictional contact with the guide link, thereby causing the guide to be abraded. This nay result in a leakage of molten resin along the abraded guide and a malfunction of the valve pin. For this reason, frequent maintenance and repair is required.